This invention relates to nuclear power reactor fuel assembly repair and more particularly to a reversible fuel assembly grid tab repair tool having a bending member for remotely reforming fuel assembly grid tabs, wherein the tool has means for fine alignment of the bending member with the grid tab to be reformed.
A nuclear power reactor is an apparatus for producing heat by the controlled fission of fissile nuclear material such as uranium-235. The nuclear material is typically formed into relatively small cylindrical fuel pellets which are stacked end to end in an elongated hollow cylindrical fuel rod which serves as cladding for the fuel rods. In the case of a pressurized water reactor, a plurality of fuel rods are bundled together by a plurality of rectangular grids each grid having generally rectangular cells therethrough for receiving each fuel rod so that the distance between fuel rods in adjacent cells obtains a predetermined pitch. The grids, which are fastened to a plurality of guide thimble tubes disposed in the fuel assembly, which thimble tubes extend the length of the fuel rods, are spaced along the length of the fuel rod bundle for securing the fuel rods in the bundle configuration by the friction engagement of a plurality of grid dimples which are integrally attached to each grid cell. A plurality of grid tabs are integrally attached to a rectangularly-shaped grid strap which wraps the outermost perimeter of the grid cells and thus binds the fuel rods in the grid. The grid tabs assist in directing cooling water flow over the fuel rods. The top and bottom of the fuel rod bundle are connected to a top nozzle and a bottom nozzle respectively for providing structural support to the fuel rod bundle. The combination of the fuel rod bundle, guide thimble tubes, grids, top nozzle and bottom nozzle form a fuel assembly. A plurality of fuel assemblies are disposed in a predetermined pattern in a nuclear reactor core which is positioned in a reactor pressure vessel. Heat due to fission of the nuclear fuel is carried away from the fuel assemblies by water circulating over each fuel assembly, which heat is transferred to a turbine for generating electricity in a manner well known in the art of nuclear power production.
On occasion, a nuclear fuel rod requires replacement prior to the end of the operating life of the fuel rod. For example, the fuel rod may experience localized rod cladding breakage. Such breakage may be caused, for instance, by a phenomenon known in the art as baffle jetting wherein high pressure coolant water jets through a deformed joint of a core baffle which surrounds the reactor core and impinges one or more of the fuel rods located near the joint. If this happens, a fuel rod near the deformed joint may experience cladding damage due to the force of the impinging water. Such a joint must be reformed and the damaged fuel rod must be replaced with an undamaged fuel rod to avoid release of radioactive material from the damaged fuel rod into the cooling water.
After the damaged fuel rod is withdrawn from its fuel assembly grid cell, an undamaged fuel rod is inserted into that cell. However, occasionally a fuel assembly grid tab, which is attached to the grid strap, is unintentionally deformed or bent when the undamaged fuel rod is inserted into a fuel assembly grid cell located adjacent the grid tab. In this manner, grid tabs may become bent from a nominal 43 degrees to as much as 90 degrees or more. When a grid tab is deformed in the manner described immediately above, the replacement fuel rod may vibrate against its adjacent deformed grid tab during reactor operation thereby increasing the likelihood that the bent grid tab may damage the fuel rod cladding. The vibration of the fuel rod is caused by the velocity and force of the coolant water flowing through and about the grid during reactor operation.
Therefore, to preclude the possible undesirable effects of such a bent grid tab, the grid tab must be reformed by bending the grid tab so that the grid tab can not vibrate against the fuel rod in a manner which can cause fuel rod cladding damage. An appropriate grid tab crimping or bending tool capable of being remotely operated may be used for this purpose.
A crimping tool adapted for crimping the edges of a panel for an automobile or the like is disclosed in U.S. Pat. No. 3,180,128, invented by O. V. Faulkner and issued Apr. 27, 1965. The Faulkner device comprises an elongated cylindrical body member having an elongated actuating member extending through the cylindrical body. A laterally projecting crimping element is carried by the actuating member in position to extend in spaced, parallel relation to a transverse plate which is carried by the cylindrical body member. The panel to be crimped is engaged between a relatively small crimping element and a relatively wide holding surface, thereby preventing damage to a finished surface which is placed in contact with the holding surface. The crimping element is urged by spring means away from the transverse plate whereby the crimping element always returns to a position to receive a member to be crimped. An adjustable stop means is also provided for varying the effective length of the actuating member. Although the Faulkner patent discloses a crimping tool most applicable to automobile panels, the Faulkner patent does not appear to disclose a tool suitable for bending or crimping a nuclear fuel assembly grid tab in the manner of the present invention which is a remotely operable and reversible fuel assembly grid tab repair tool.
U.S. Pat. No. 4,614,106 issued Sept. 30, 1986 to D. Forget and entitled "Tab Lifting And Crimping Tool" discloses a tab lifting tool suitable for use on radiators of a type commonly used to cool motor vehicle engine cooling water. The tool includes a casing having a pair of openings at opposite ends thereof and an elongated tool channel inside the casing which communicates with the openings. An elongated tool member slidably mounted in the tool channel has a tab lifting finger proximate a tab lifting end of the casing and a tab crimping face proximate a tab crimping end of the casing. A flange gripping finger is affixed to the tab crimping end of the casing while a trigger is pivotally mounted in the casing. Biasing means are also located in the casing and engage the tool member for urging the tab lifting finger away from the casing and the tab crimping face towards the casing. Even though the Forget patent discloses a tool for lifting and crimping automobile radiator tabs, the Forget patent does not appear to disclose a tool for remotely bending or crimping a nuclear fuel assembly grid tab in the fashion of the present invention.
U.S. Pat. No. 3,570,299 issued Mar. 16, 1971 to A. W. Wieters and entitled "Internal Duct Crimper" discloses a crimping device for bending and clenching edges of laterally extending ducts to main ducts including a bifurcated arm structure with a clenching device and anvil provided on the end of one of the arms with means for shifting the clenching member into clenching position against the anvil through the utilization of a connected arm and lever device. The apparatus provides a duct crimper wherein the crimping mechanism, including a shoe and anvil device, is carried completely on one leg of the unit. Although the Wieters patent discloses a tool for bending and clenching edges of laterally extending ducts to main ducts, the Wieters patent does not appear to disclose a reversible device configured for remotely bending or reforming nuclear fuel assembly grid tabs as provided by the present invention.
A remotely operable device for studying fission gases generated within fuel rods during reactor operation is disclosed by U.S. Pat. No. 4,428,903 issued Jan. 31, 1984 in the name of James E. Kasik et al. and entitled "Fuel Rod Fission Gas Crimping Arrangement And Method". To study these gases, irradiated fuel rods are individually punctured in a subaqueous environment to release the fission gases for capture and examination. In this regard, a selected fuel rod is withdrawn from its bundle for penetration. After withdrawal from the bundle, and after penetration and capture of fission gas, the selected fuel rod is stored underwater for an indefinite period of time. The Kasik et al. device remotely and sealingly crimps a malleable sleeve over the puncture hole through which fission gases escape in order to prevent further release of fission gasses when the fuel rod is stored. According to the Kasik et al. disclosure, a hydraulically operated crimping mechanism forms a pair of sealing ridges (one on each side of the puncture hole) between the malleable sleeve and the punctured fuel rod by crimping the malleable sleeve with its jaws. At the appropriate moment, the jaws clench, securing the malleable sleeve onto the fuel rod and over the puncture hole. Although the Kasik et al. patent discloses a remotely operable crimping tool suitable for use on a nuclear fuel assembly component (e.g., a nuclear fuel rod), the Kasik et al. patent does not appear to disclose a reversible tool having a bending means for reforming a fuel assembly grid tab in combination with a spring biasing means connected to the bending means and does not appear to disclose an anvil surface on the tool against which the grid tab can be bent or reformed. Moreover, the Kasik et al. patent does not appear to disclose a device having means for fine alignment of a bending mechanism with the grid tab to be reformed.
Consequently, while the prior art discloses crimping devices for crimping elements or tabs, the prior art does not disclose a reversible device having a bending member which is suitable for remotely bending or reforming a fuel assembly grid tab, wherein the device has means for fine alignment of the bending mechanism with the grid tab to be reformed.
Therefore, what is needed is a reversible fuel assembly grid tab repair tool having a bending member for remotely reforming a fuel assembly grid tab, wherein the repair tool has means for fine alignment of the bending member with the grid tab to be reformed.